Waterfall ice: mechanical stability of vertical structures

摘要

We present a study of the mechanical (in)stability of the ephemeral waterfall ice structuresthat form from the freezing of liquid water seeping on steep rock. Three vertical structures were studied,two near Glacier d'Argentiere, France, and one in the Valsavarenche valley, northern Italy. Thegeneration of internal stresses in the ice structure in relation to air- and ice-temperature conditions isanalyzed from pressure sensor records. Their role in the mechanical instability of the structures isdiscussed from a photographic survey of these structures. The main result is that dramatic air cooling(several °C h~(-1)over several hours) and low temperatures (<-10°C), generating tensile stresses andbrittleness, can trigger a spontaneous or climber-induced mechanical collapse, leading to unfavorableclimbing conditions. Ice internal pressure fluctuations are also associated with episodes of markeddiurnal air-temperature cycle, with mild days (few °C above 0°C) and cool nights (few °C below 0°C),through the occurrence of water→ice phase transitions within the structure. These ice internal stressfluctuations seem, however, to have a local influence, are associated with warm (near 0°C), wet andtherefore particularly soft ice and do not trigger a collapse of the structure.